This invention relates to perpendicularly orientable nematic compounds and to the use thereof in nematic compositions and in electronic components.
Many uses of nematic compounds and compositions have been described, e.g., App. Physics Letters 13: 46 (1968) and Scientific American 222: 100 (1970), the contents of which are incorporated by reference herein; and Zeitschrift fur Naturforschung 20a: 572 (1965); 23a: 152 (1968); Osterr. Chem.-Ztg. 68: 113 (1967) (use of nematic liquid crystals in nuclear resonance spectroscopy).
Nematic substances are compounds or mixtures of compounds capable of forming an enantiotropically nematic phase, i.e., their transition point from the anisotropic to the isotropic condition (clearing point) is above their melting point. Substances which are monotropically nematic, i.e., wherein the transition point from the ansiotropic to the isotropic condition is below the melting point in the metastable range, are called nematogenic, which form an enantiotropic nematic phase only in a mixture with other nematogenic or nematic compounds.
In the development of electronic components, particularly electronic indicating devices, characterized inter alia by a flat structure and richness in image contrast compared to conventional counter or cathode-ray tubes, liquid crystals with a nematic phase have served for several years as picture screen material. These compounds display, in their nematic range i.e., the range between their melting point (m.p.) and their transition point (t.p.) a change in their light-scattering characteristic which can be controlled by AC or DC electrical fields. A prerequisite of this property, called dynamic scattering effect, is that the dipole moment of the molecule forms an angle with the longitudinal axis of the molecule.
In order to exploit this dynamic scattering effect for the production of images, a thin layer of a thickness of a few microns of a suitable nematic compound or compound mixture is placed between two electrode plates, one or both of which are transparent. If an electrical field is now applied, contrasts are produced by the change in light dispersion which can then be observed by direct frontal view or by rear viewing.
Nematic phases are utilized for converting electrical voltage pulses into optical pulses. Normally, a thin layer of liquid crystals in embedded between translucent or transparent plates having a conductive coating. An electrical voltage at the two electrodes generates in the nematic layer a microscopically visible, strong turbulence connected with a macroscopically visible intense increase in the light scattering of the electrode unit. During this process, the system becomes cloudily opaque.
The great advantage inherent in this type of electrooptic device could only be really utilized upon the discovery of nematic plates distinguished by great stability in AC and DC electrical currents and by a high chemical stability. In the meantime, a number of such nematic phases have become known, especially also those having very low-melting, wide nematic mesophase ranges.
Normally, nematic phases in a thin layer between glass or electrode plates form streaky and unoriented schlieric layers, resulting in decreased contrast of the indicator units and an unattractive appearance. Arranged in a thin layer between two glass plates or electrodes, the molecules of such phases have several possible modes of orientation. If all molecules are arranged with their longitudinal axes perpendicular to the electrode surfaces, the display device, without the application of electrical voltage, appears entirely transparent and glass-clear from any angle of observation. If the longitudinal axes of the molecules forming the nematic phase are all uniformly parallel to the glass surfaces, then transparent indicating systems are obtained when viewed from an angle of about 90.degree.. However, at obtuse observation angles, e.g., of 45.degree. or less, such layers exhibit a slightly cloudy appearance. In nematic phases containing azoxybenzene derivatives, this phenomenon is particularly disadvantageous because the color of the display device, dependent on the inherent yellow color of the azoxybenzene derivatives, is measurably more intense in case of a parallel orientation of the molecules than in the case of a perpendicular orientation. It is furthermore possible for the preferred direction of the molecules to be different at different locations of the display devices. In such a case, the display devices exhibit, at any angle of observation, a schlieren-permeated, irregularly turbid appearance.
Although the orientation of nematic phases between the electrodes is not decisive for the finite functioning of a display unit, perpendicularly oriented layers are preferred since, after the application of electric fields, these yield substantially better contrast under any angle of observation with respect to the areas not under voltage than do comparable nematic layers which are parallel-oriented or even inhomogeneously oriented with respect to the electrode surfaces. A process for the perpendicular orientation of nematic substances is described in U.S. Pat. application Ser. No. 334,603 filed Feb. 22, 1973.
Upon the application of an operating voltage to indicator elements filled with a perpendicularly oriented nematic liquid, a certain time period is required for turning the molecules from the vertical position into a position wherein the longitudinal axes of the molecules are arranged parallel to the electrode surfaces, since only after this has been accomplished does the dynamic scattering effect occur. The time required for this molecular orientation is prolonged at a given voltage under the influence of ultraviolet radiation, i.e., UV light undesirably increases the threshold voltage necessary for dynamic scattering, particularly when nematic substances of perpendicular orientation are employed in the indicator device.